There are many occasions during the inspection and repair of pipelines when it is necessary to seal off the bore of the pipeline at a given location. For example, a pipeline may require sealing at a certain point in order to patch a leak or allow replacement of a section. In other instances, the pipeline bore may be sealed and the pressure raised behind the sealing device in order to ascertain if a leak exists in a particular pipeline section, the pressure of such leak being indicated by a failure of the pipeline to hold pressure. Several prior art approaches have been taken in the design of such sealing devices.
U.S. Pat. No. 3,593,749 discloses a pipeline pig stopper which relies on mechanical grippers or slips to fix the pig in the pipeline bore, after which an inflatable element is expanded to effect the actual seal. Major disadvantages of such a device include the limitation of gripping power due to the relatively small size and rigid configuration of the grippers, and damage incurred by the interior of the pipe in which the grippers are set. These grippers may not conform exactly to the pipeline bore wall, which may be lined with relatively soft plastics. Such plastics can be easily damaged by the gripper serrations, particularly if the grippers are not perfectly aligned with the bore wall.
U.S. Pat. No. 4,077,435 discloses a pipeline plugging apparatus which relies on stopping mechanisms inserted in the wall of the pipeline to arrest plug movement, subsequently sealing the pipeline bore with an expandable elastomer seal. While effective, the use of such stopping mechanisms necessarily limits the use of such plugs to instances where the pipeline is easily accessible and where a leak has already been located.
U.S. Pat. No. 4,079,755 discloses a rubber-covered inflatable pipeline plug reinforced with tire cord to withstand high inflation pressures. The utilization of an elastomer necessarily limits the anchoring force obtainable by the device, as the elastomer will tend to "creep" at high pipeline pressures, acting like a highly viscous fluid. Such "creep" is extremely undesirable, particularly when an exact pipeline location is desired for repair or leak location purposes.
In addition to the disadvantages noted above, the prior art devices also are extremely limited in their ability to traverse curves in a pipeline, unless they are made extremely short relative to their diameter, and a number of anchors or plugs employed in tandem. This is due in part to the limited expandability of the prior art devices, necessitating outer diameters which place the unset gripping or anchoring means close to the pipeline wall, and in part due to the fact that these devices are of a relatively constant diameter throughout their length.